The injection moulding of a thermoplastic resin has heretofore been directed generally to a technique characterized by moulding a resin mixture in a mould utilising the plasticity of a thermoplastic resin. The thermoplastic resin is melted, injected into a mould and then cooled and solidified in the mould under a static holding pressure to obtain a desired moulded article. Recently, a great number of thermoplastic resins have been commercialised in response to various required characteristics.
However, since a moulded article having more and various functions has been required, a single material has become unable to respond to the requirement in some cases. To solve such a problem, in the resin processing field, multilayer moulding processes utilising the characteristics of various resins such as co-extrusion in extrusion moulding, lamination moulding in film formation, multilayer blow moulding in blow moulding and the like have been developed as processes for satisfying the multifunction characteristics. Also, in the field of injection moulding, there has been used a sandwich moulding method in which two kinds of resins are plasticized in two separate cylinders and simultaneously injected into a mould through a single sprue or runner to prepare a double layer moulded article. Moreover, there has been used a two-colour moulding method in which a moulded article formed previously in a mould is placed into another mould and then a different type of a molten resin is injected thereto to form a composite article. There has also been used an insert moulding method in which a metal material, a film or the like previously prepared by a method other than injection moulding is inserted into a mould and a molten resin is filled to a cavity formed between the mould and the insert to form a composite article.
In the case of the above-mentioned multilayer extrusion moulding method, multilayer blow moulding method and multilayer film-forming method, there is a limit in the preparation of a moulded article having a complicated shape, and the moulded article is limited to flat plate, hollow article, film and the like. On the other hand, in the case of the sandwich moulding method, the productivity of the injection moulding per sc is maintained; however, a sophisticated technique is required for controlling the relative thickness of a skin layer and a core layer in the preparation of a double layer moulded article by moving two kinds of resins in the same direction, and there is a great restriction in respect of the combination of materials which can be used for controlled multilayering. In general, when a sandwich moulded article having a uniform skin layer is prepared, it is necessary to adjust the fluidity of the two resins and it is difficult to prepare a multilayer moulded article from materials having extremely different fluidity. Moreover, in the case of the sandwich moulding method, a double layer moulded article can be prepared, but a moulded article having three or more layers is difficult to prepare. In the case of the insert moulding method, there is no such great restriction as in the above-mentioned sandwich moulding as to the combination of materials; however, a plurality of moulds are required and such a procedure is required that a previously prepared material must be inserted into a mould. Therefore, the insert moulding method is inferior in productivity to conventional injection moulding and conventional sandwich moulding methods. In general, the moulded article is a two-layer moulded article, and when it is intended to prepare a moulded article having three or more layers, a further mould or moulds and a further procedure or procedures corresponding to the number of layers are required for the preparation. Therefore, at present, the insert moulding method is poor in practicability.
In the case of the conventional injection moulding method, it follows that a melt of a plastic material is injected into a mould from one resin-feeding site and then a considerable packing force is applied to the single feeding site.
U.K. Patent No. 2,170,142 and Japanese Patent Application Kokoku No. 4-3,893 disclose a method which comprises applying a shearing force to the molten material charged into the mould in the course of solidification. When this technique is used, the microstructure of the interior of the moulded article can be substantially controlled, whereby it becomes possible to enhance the mechanical characteristics of the moulded article. Certainly, the use of this technique is effective for control of the micromorphology of the interior of the moulded article, and brings about, a great effect on the strengthening of the weldline and the overcoming sink marks in a thick-wall moulded article. However, in the case of the above-mentioned method, the preparation of a multilayer moulded article composed of different materials by applying a shearing force to different resins in a mould has not been carried out, and is not simple.
As regards the sandwich moulding method in which two different resins are moved in the same direction to obtain multilayer mouldings, EP-A-0191623 discloses such a technique that a filling force is periodically given to act on the molten resins to prevent sink marks and the like. However, it has been very difficult even by this method to obtain a multilayer moulded article in which each layer has a controlled thickness.
As a method of preventing a resin mixture from solidifying at too early stage, it has been proposed to elevate the temperature of a mould. However, when the temperature of a mould is elevated, such results are generally brought about with the disadvantage that long cooling times become necessary, and the product is taken out of the mould before complete solidification and a moulded article having a very poor dimension precision is obtained. Therefore, in the actual moulding, a compromise temperature between the two contradictory conditions is selected. As one effective injection moulding technique for solving these problems, Japanese Patent Application Kokoku No. 58-40,504 and U.K. Patent No. 2,081,171 proposes a method which comprises previously heating the mould surface by high-frequency induction heating.
However, according to this method, the microstructure of the interior of a moulded article has been difficult to control though the skin layer of the moulded article can be controlled.
This invention can provide a moulding process by which a multilayer moulded article whose fine structure is controlled is obtained, and also a multilayer moulded article whose fine structure is controlled.